This invention concerns a method for the static and dynamic control of the planarity of flat rolled products, such as strip or similar. The method is advantageously applied in five- or six-high stands, having a pair of working rolls (WR) associated with both negative and positive bending mechanisms and axial displacement, or shifting, mechanisms, a pair of back-up rolls (BUR), and at least an intermediate roll (IR) associated with a crossing mechanism and a positive and negative bending mechanism.
The method to control planarity provides that the second order components, the fourth order components and the edge-drop of the profile of the rolled strip are controlled independently. This control may occur both statically, (that is, during the initial setting or preset of the rolling mill, to set up the stand before rolling starts, to take it to adequate working conditions) and also dynamically, during rolling. To be more exact, the second order and fourth order components can be dynamically controlled, and even controlled with great efficiency.
The state of the art includes a method to control the planarity of flat rolled products in six-high rolling stands, wherein both the working rolls and the intermediate rolls are associated both with bending systems, both negative and positive, and also with a system of long axial translation or shifting (macro shifting).
This method of control, however, has the disadvantage that it cannot completely and efficiently compensate edge-drop, and requires a particularly long axial translation of the intermediate rolls.
Another disadvantage of this method, which provides for the shifting of the intermediate rolls, is that the speed at which the shifting is performed is extremely slow compared with the rolling speed, that is to say, about 1/1000 of the latter. Therefore, if the setting of the stand is not correct, since the inlet profile of the product being rolled is different from the final profile, or the rolling force is different from the initial expected one, there is a delay in the re-setting of the stand, for example because of the speed of shifting, with a resulting loss of planarity for a length of strip which is equal to the time taken to reset the stand multiplied by the rolling speed.
To at least partly solve this problem of compensating the edges, various rolling stands and control systems have already been proposed which provide a system of axial translation of the working rolls in the same direction as the intermediate rolls and wherein the working rolls are equipped with appropriate bevels or hollows at the ends.
Moreover, the state of the art also includes a rolling method wherein the intermediate rolls (IR) are associated with crossing means suitable to reduce the so-called xe2x80x9cstrip walkingxe2x80x9d.
The present Applicant has devised, designed and perfected the method to control the planarity of rolled products according to the invention to overcome the shortcomings described above and to improve the methods known in the state of the art.
One purpose of the invention is to achieve a method for the static and dynamic control of the planarity of flat rolled products, such as strip or similar, which will make possible to control and adjust, autonomously and independently, both statically and dynamically, that is to say, during rolling, both the x2 component and the x4 component, but also components of a higher order, which consequently makes it possible to control the edge-drop of the rolled product, that is to say, components up to x10. 
In accordance with this purpose, the method for the static and dynamic control of the planarity of flat rolled products according to the invention comprises a pair of working rolls, a corresponding pair of back-up rolls and at least an intermediate roll located between one of the working rolls and a corresponding back-up roll, shifting means and bending means associated with at least one of the working rolls to translate it axially and respectively bend it, and crossing and bending means associated with the intermediate roll to arrange it with its longitudinal axis inclined, or rotated, with respect to the longitudinal axes of the working rolls and the back-up rolls and respectively bend it.
Before describing the invention in detail, it is appropriate to make the following premises:
The ability to control the profile of the strip being rolled is generally shown in the plane x2, x4 (FIG. 5), where x2 and x4 are the second and fourth order components of the function y(x)=a0+a1x+a2x2+a3x3+ . . . +a10x10, which represents the thickness of the strip (FIG. 6).
If the thickness is symmetrical, as it should be, the odd components should not be present. At most, we might find the component a1x which indicates the presence of strip with a wedge defect, that is, a profile which is on average trapezoid with edges of a different thickness, as shown in FIG. 7.
The more efficient a stand is at controlling the shape, the wider is the zone x2, x4 which can be controlled; FIG. 8 shows two areas, the most extensive of which refers to a system with a higher control capacity than the more inward area.
If a stand has high dynamic performance in controlling the shape of the strip, this means that it is possible to pass quickly from a point A (FIG. 9) to a point B in the plane x2, x4. Then, together with an area of xe2x80x9cstaticxe2x80x9d or preset control, an area of xe2x80x9cdynamicxe2x80x9d control is also shown, clearly included in the area of static control which moves inside the area of global control (FIG. 10) according to the initial static functioning point xe2x80x9c0xe2x80x9d.
Since every actuator suitable to control the movements of the working rolls and intermediate rolls, in every operating condition (that is, roll diameters, strip width, inlet profile, rolling force, etc.) has its own xe2x80x9cline of actionxe2x80x9d, to pass with complete freedom from a point A to a point B, it is generally necessary to have two actuators AT1 and AT2 which move in their own directions d1 and respectively d2 (FIG. 11). Therefore, in the field of dynamic control, to have the possibility to pass from A to B without constraints on position B, the two necessary actuators must also have lines of action which are not parallel.
This having been said, FIG. 12 shows the control of the crossing of an intermediate roll (IR) according to the invention, wherein it can be noticed how the influence of x2 has limited collateral effects on x4, since the ratio between x2 and x4 is about {fraction (1/10)}. Therefore, by acting on IR crossing we have very limited effects on the x4 component.
From the detail shown in FIG. 13, in which the two working rolls (WR) are shown, it can be seen how WR shifting prevalently influences the edges of the strip, if the working roll is appropriately bevelled.
WR shifting influences both x2 and x4 but in a very limited way compared with WR bending, IR crossing and IR bending. WR shifting is practically defined by the width of the strip, with very small adjustments according to the actual edge-drop on the strip at outlet. The ratio between x2 and x4 is about 1.
As can be seen in FIG. 14, WR bending influences both x2 and x4. The ratio x4/x2 depends on the choice of the diameters of the rolls of the stand and on the width of the strip (rolling force, etc.), and is in any case near 1.
In FIG. 15 it can be seen how IR bending prevalently influences x2 with collateral effects on x4 (as for IR crossing), even though the action is less efficacious than that obtained with IR crossing. The x4/x2 ratio is about {fraction (1/10)}.
The influence of IR crossing, IR bending, WR bending on the edges of the strip is very limited, and therefore when IR crossing, IR bending and WR bending is varied, it is not necessary to modify the set of WR shifting.
Therefore, the rolling stand which adopts the method according to the invention is equipped with means which allow IR crossing, IR bending, WR shifting and WR bending.
To be more exact, WR shifting is used to pre-set the working rolls according to the edge-drop. This constitutes a xe2x80x9cstaticxe2x80x9d actuator which does not influence the field of control x2/x4 since it is constrained only to the desired edge-drop correction.
IR crossing is used to pre-set the IR to obtain a desired x2 component. IR crossing is obtained by means of a preset actuator which can however be used in rolling too, to change the x2 component if the other actuators which control x2 dynamically (that is, WR bending and IR bending) are near saturation.
WR bending and IR bending are dynamic controllers and generally have to act simultaneously if it is desired to correct a x2, x4 defect during rolling (FIG. 16). Moreover, WR bending and IR bending must have the same dynamic performance, with reply times of less than tenths of a second, and have to act simultaneously. See for example the graphs in FIG. 17 and FIG. 18, which show a dynamic compensation x2 and respectively a dynamic compensation x4. For this reason, both WR bending and IR bending must be able to be both positive and negative.
With respect to a conventional rolling stand, or equipped with WR bending, IR bending and IR shifting, it is appropriate to make the following considerations.
IR shifting, as in conventional stands, practically has an influence only on x2 (the x4/x2 ratio is equal to about {fraction (1/15)}), and has an action of x2 variation reduced by about 3-4 times compared with those of IR crossing. The comparison is between IR shifting with a travel of 200 mm and IR crossing with a rotation of 0-1.5xc2x0. Therefore IR crossing is much more efficient.
Moreover, IR shifting, where it is included, is variable in rolling, with shifting speeds of {fraction (1/1000)} of rolling speeds to prevent damage to the surfaces of the rolls. With a rolling speed of 20 m/sec we have a shifting speed of 20 mm/sec. Therefore, it would take 10 secs to carry out the whole control travel.
The IR crossing speed is higher, at about 0.1xc2x0/sec. Consequently, to have the same x2 variation corresponding to the whole shifting travel (in the embodiment which includes IR shifting), it is enough to vary the crossing angle by 0.2-0.6xc2x0, according to the starting point (FIG. 19).
Moreover, crossing is quicker: 0.2-0.6xc2x0 are varied in 2-6 secs, whereas with IR shifting it needs at least 10 secs to carry out the whole travel and obtain the same effects on the strip.
Thanks to the high control capacity of IR crossing, which as we have seen is on average about three times more than that of IR shifting in conventional stands, it is possible to use IR crossing also in a five-high stand, keeping high the capacity to control the profile of the strip.
According to a preferential embodiment of the invention, a pair of intermediate rolls is located between the pair of working rolls and the pair of back-up rolls, therefore the rolling stand is the six-high type.
According to a simplified variant, only one intermediate roll is arranged in the upper section between a corresponding working roll and a corresponding back-up roll, therefore the stand is of the five-high type.
According to one characteristic of the invention, the bending of each working roll and intermediate roll can be both positive and negative.
According to another characteristic of the invention, the working rolls are provided, at least at one end, with bevels appropriately configured so as to control the profile of the edges of the rolled product.
According to another characteristic of the invention, the crossing mechanism allows to carry out the crossing of each intermediate roll quickly, during the rolling step, since the maximum rotation of the intermediate rolls, compared with the working rolls, is about 1.5xc2x0 and since the speed of rotation is about 0.1xc2x0/sec, the correction operation, which requires to vary the angle by 0.2-0.6xc2x0, is carried out in about 2-6 secs.
According to another characteristic of the invention, the method to control the planarity of flat products provides a step of monitoring, by sensor means, the profile of the product emerging from the stand, and a step of acting on shifting means and bending means associated with at least one of the working rolls to translate it axially and respectively bend it, on crossing means and bending means associated with the intermediate roll to arrange it with its longitudinal axis inclined, or rotated with respect to the longitudinal axes of the working rolls and the back-up rolls and respectively bend it.
With reference to FIG. 20, in which xe2x80x9c0xe2x80x9d indicates the work point which represents the profile of the strip with all the actuators in the inactive position, or initial position, and xe2x80x9cFxe2x80x9d indicates the point which represents the strip profile desired, it should be remembered that to obtain the desired profile it is necessary, according to the invention, to make the following operations:
Set WR shifting to suitably correct the edge profile (xe2x80x9cedge-drop compensationxe2x80x9d); from point xe2x80x9c0xe2x80x9d we pass to an intermediate point xe2x80x9c1xe2x80x9d;
Set IR crossing to modify the x2 component in preset mode;
Act dynamically on IR bending and WR bending to move on the x2, x4 plane and reach the required xe2x80x9cFxe2x80x9d point.
Where the use of IR bending and WR bending is not enough to reach point xe2x80x9cFxe2x80x9d, activate IR crossing dynamically to quickly reach the desired performance.